Lost material textures in Blender 2.83
You can have Blender attempt to locate the missing files under the external data section of the File menu. See image below:

If, on the other hand, you need to reconnect the textures manually, you can do so by selecting the material and going to the SHADING tab. From there, if there is no image texture, add one, and connect it as in the image below. This example is a very basic setup for a single image. If your material is PBR and there are other texture maps, add them and connect them to their respective inputs on the Principled BSDF. NOTE - make sure to select a non-color space for Normal, Displacement, Metallic and Roughness maps.